Conventionally, in a wireless LAN system, another base station is searched by a wireless LAN terminal for selection as a switching candidate such that even if the wireless LAN terminal moves and cannot communicate with a base station with which it is currently communicating, it can continuously communicate with the other base station. This search method continuously observes the signal intensity or the communication quality of a signal transmitted from the base station with which the wireless LAN terminal is currently communicating and which is received at the wireless LAN terminal. Then, when the observed signal intensity or communication quality falls below a previously set base station search start trigger threshold, the wireless LAN terminal searches for another base station which is selected as a switching candidate.
Also, a method has been contemplated to change the aforementioned base station search start trigger threshold in accordance with the reception level at a terminal of a radio wave transmitted from another base station which is to become a switching candidate (see, for example, JP-2001-145146-A).
Also, a method has been contemplated to set a long scan interval for searching another base station which is to be a switching candidate if there are no other base stations exist from which a switching candidate can be selected (see, for example, JP-2003-108271-A).
However, in the method which uses a previously set base station search start trigger threshold, since the base station search start trigger threshold is single, a futile search can be made depending on the base station installation situation around the location at which a wireless LAN terminal exists. For example, even though no other base stations are installed in surroundings, another base station may be searched in some cases for use as a switching candidate only under the condition in which the signal intensity and the communication quality of a signal, transmitted from a base station with which communication is currently in progress and which is received at the wireless LAN terminal fall below the previously set base station search start trigger threshold. This gives rise to a problem in which power, which is often battery driven, is wasted when used in a wireless LAN terminal.
FIG. 1 is a diagram showing an example in which a wireless LAN terminal moves to a position at which a base station search start trigger threshold is exceeded in a conventional wireless LAN system.
In the conventional wireless system shown in FIG. 1, assume that base stations 900-1-900-2 cover communication areas 902-1, 902-2, respectively, and terminals 901-1-901-2) which are wireless LAN terminals, are communicating with base station 900-1. Assume also that a base station search start trigger threshold for terminals 901-1-901-2 to search another base station is threshold 903 in area 902-1. Assume also that terminal 901-1 exists at position (r), while terminal 901-2 exists at position (t). In this event, since terminals 901-1, 901-2 are positioned within threshold 903, terminals 901-1-901-2 do not search another base station. Here, threshold 903 schematically represents a threshold for the signal intensity and communication quality of a signal transmitted from a base station and received at the wireless LAN terminals.
Subsequently, assume that terminal 901-1 has moved from position (r) to position (s), while terminal 901-2 has moved from position (t) to position (u). As a result, since terminals 901-1-901-2 have gone out of threshold 903, terminals 901-1-901-2 search another base station. With respect to terminal 901-2, since it exists within area 902-2 covered by base station 900-2, base station 900-2 is detected as a switching candidate. On the other hand, with respect to terminal 901-1, since it does not exist within an area covered by another base station, any base station is not detected. Since another base station is subsequently searched at regular intervals as well, a problem arises in that the terminal performs the search processing even though it exists at a location at which any base station cannot be searched, causing useless power consumption.
On the other hand, in the method described in JP-2001-145146-A, when a terminal exists at a position at which a radio wave, transmitted from another base station which is to be a switching candidate, does not reach, i.e., at position (s) shown in FIG. 1, the terminal searches a base station at the same frequency, causing a problem in which power is wasted, as is the case with the aforementioned problem.
Also, in the method described in JP-2003-108271-A which simply adjusts the scan time for the search, processing for searching a base station is performed even when a terminal exists at a position at which a radio wave, transmitted from another base station which is to be a switching candidate, does not reach, i.e., at position (s) shown in FIG. 1, causing a problem that power is wasted, as is the case with the aforementioned problem.